1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying apparatus, a printer, and the like and, more particularly, to a transfer type image forming apparatus for electrostatically transferring a developing agent image formed on an image carrier onto a transfer medium.
2. Description of the Related Art
Various image forming apparatuses, each of which forms a toner image as a developing agent image on a photoconductive drum by, e.g., an electrophotographic method, and electrostatically transfers the toner image onto a transfer sheet, have been proposed and put into practical applications.
Normally, in these image forming apparatuses, a toner image formed on the photoconductive drum is electrostatically transferred onto a transfer sheet by corona discharge from a transfer charger, as disclosed in U.S. Pat. No. 4,893,146, for example. Thereafter, the transfer sheet on which the toner image is transferred is electrostatically separated from the photoconductive drum by corona discharge from a separation charger, which is arranged adjacent to the transfer charger.
A convey guide having a plurality of rib-like members is arranged to oppose the separation charger. The transfer sheet separated from the photoconductive drum is guided by the convey guide and prevented from entering the separation charger.
However, in the above-mentioned conventional apparatus, the material of the convey guide is determined regardless of the charged polarity of a developing agent, and the convey guide is triboelectrically charged upon contact with a transfer sheet. In this case, an electric charge electrified on the convey guide disturbs the developing agent image transferred on the transfer sheet, thus often causing an image defect. Since the convey guide is constituted by arranging the plurality of rib-like members at predetermined intervals, when, for example, a halftone image is transferred onto the entire surface of the transfer sheet, blank portions are formed on the image in correspondence with that portions of the transfer sheet which contact the rib-like members. As a result, an image defect occurs, that is, blank lines extending parallel to the travel direction of the transfer sheet appear on the image in correspondence with the number of the rib-like members. In particular, this tendency is conspicuous in a low humidity environment state wherein the transfer sheet has a high resistance.